The present invention primarily concerns the fabrication of articles by rotomolding, also called rotational molding. In rotomolding a premeasured amount of polymer is placed in one half of the mold, the mold is closed and then heated until the polymer is molten. The mold is rotated so as to get good distribution of the polymer in the mold. The mold can be rotated either uniaxially or biaxially, but biaxial rotation is widely preferred, i.e. simultaneous rotation around two perpendicular axes. In the following the mold is cooled, opened and the formed article is removed from the mold. Rotomolding can also be used for multilayer molding, for example by using more than one polymer sequentially. Rotomolding allows the production of hollow articles with good wall thickness distribution and good mechanical properties.
Slush molding is a process that is closely related to rotomolding. In the following the term rotomolding is therefore used to imply both, rotomolding and slush molding applications.
The most widely used polymer in rotomolding is polyethylene. Therefore a lot of effort has been invested to improve the processability of polyethylene in rotomolding.
U.S. Pat. No. 6,362,270 discloses polymer compositions particularly suited for rotomolding. These polymer compositions comprise of at least 94% by weight of one or more thermoplastic polymers and a maximum of 6% by weight of one or more processing additives. The thermoplastic polymer may be selected from copolymers of ethylene and styrene, ethylene and/or C3-C20 alpha-olefin homo- or copolymers, nylon, polyethylene terephthalate, polycarbonate, acrylic polymer, polystyrene, and blends of these polymers. Suitable processing additives include aromatic or aliphatic hydrocarbon oils, esters, amides, alcohols, acids, and their organic or inorganic salts as well as silicone oils, polyether polyols, glycerol monostearate (GMS), pentaerytritol monooleate, erucamide, stearamides, adipic acid, sebacic acid, styrene-alpha-methyl-styrene, calcium stearate, zinc stearate, phthalates and blends thereof. The processing additive preferably decreases the composition's melt viscosity and/or elasticity at zero or low shear rates to allow for a reduction in sintering time, cycle time and/or maximum mold temperature.
A recent report (L. T. Pick, E. Harkin-Jones, Third Polymer Processing Symposium, Jan. 28-29, 2004, Belfast, p. 259-268) shows a correlation between the number of bubbles in a rotomolded article and its impact performance, with a higher number of bubbles resulting in lower impact performance. In addition, a high number of bubbles has a negative influence on the optical properties of the finished articles.
There is thus a need to provide a rotomolded article with a reduced number of bubbles.
There is also a need to provide a rotomolded article with improved optical properties.
There is also a need to provide a rotomolded article with improved mechanical properties.
It is an object of the present invention to provide a rotomolded article based on a polyolefin composition with improved processability in rotomolding applications.
It is another object of the present invention to provide a rotomolded article based on a polyolefin composition that improves the sintering and densification processes in rotomolding applications.
It is another object of the present invention to provide a rotomolded article based on a polyolefin composition that reduces bubble formation in the rotomolding process.
It is another object of the present invention to provide a rotomolded article based on a polyolefin composition that improves optical properties of the articles produced by rotomolding applications.
It is another object of the present invention to provide a rotomolded article based on a polyolefin composition that improves mechanical properties of the articles produced by rotomolding applications.
It is another object of the present invention to provide a rotomolded article based on a polyolefin composition that allows reducing cycle time in a rotomolding process.
It is another object of the present invention to provide a process for producing by rotomolding an article with improved properties.
It is yet another object of the present invention to provide a rotomolding process with improved densification and/or sintering processes.